Ion implantation has become a standard technique for introducing impurity dopants into semiconductor wafers. A beam of ions is generated in a source and is directed with varying degrees of acceleration toward a target wafer. Ion implantation systems typically include an ion source, ion optics for removing undesired ion species and for focusing the beam, means for deflecting the ion beam over the target area and an end station for mounting and exchanging wafers.
In the implantation of semiconductor wafers for large scale integrated circuits, it is required that the implanted dose be carefully controlled in magnitude and be highly uniform over the area of the wafer to insure uniform and predictable device characteristics. The ion beam used to implant the wafer has a cross-sectional dimension of up to 5 inches and exhibits a variation in intensity over its cross-sectional area. In rotating disc or spoke systems where a relatively large area beam is utilized and the area element being implanted is a function of radius, a knowledge of the beam distribution pattern is particularly important. If the radial intensity centroid of the ion beam is displaced from the nominal beam axis, the integrated dose distribution pattern is displaced relative to its nominal location and both the sheet resistance and the uniformity of the implanted wafer are adversely affected. Therefore, it is important to know the location of the intensity centroid of the ion beam.
In the past, the intensity centroid of the ion beam in ion implantation systems has not been determined. However, various diagnostic devices have been used in charged particle acceleration systems to determine the shape of the beam distribution. A moving wire has been used to measure beam current, either directly through the wire, or by secondary electron current measurement. Current has been measured behind a moving slit or aperture. Individual Faraday cups for measuring current have been placed at different locations in the beam or behind an aperture plate. The beam has been scanned over one or more apertures or slits. A micro-calorimeter has been used to measure beam intensity, either with the micro-calorimeter being moved, or with the beam being scanned over the micro-calorimeter. Also, the beam can be located by optical omission from the beam hitting a surface. These techniques have several problems and disadvantages. Any of the techniques requiring either a moving beam or a moving beam monitor require an accurate motion control device and introduce mechanical complexity. Mechanical devices are undesirable within the vacuum chamber of an ion implanter because they tend to introduce particulate contamination. Individual Faraday cups do not require mechanical motion but, in the past, have required either multiple vacuum feedthroughs for conductors from each Faraday cup or active electronic circuitry within the vacuum chamber. Such electronic circuitry may cause contamination and outgassing problems within the chamber.
It is a general object of the invention to provide improved methods and apparatus for charged particle beam centroid location.
It is another object of the present invention to provide methods and apparatus for charged particle beam centroid location requiring no moving parts.
It is a further object of the present invention to provide methods and apparatus for charged particle beam centroid location which minimize the possibility of contamination within a vacuum chamber.
Yet another object of the present invention is to provide methods and apparatus for charged particle beam centroid location requiring only two electrical feedthroughs between the vacuum chamber and the external environment for the centroid location function.